Like a ship passing through water, the solar system plows through interstellar space in this artist's conception.

Particles streaming from the sun create solar winds, which form a bubble around the solar system dubbed the heliosphere.

At a point in this bubble called the termination shock, the solar winds abruptly slow are affected by interstellar winds. In 2004 the Voyager 1 spacecraft crossed the termination shock and entered the heliosheath, the solar system's outmost layer.

Voyager 2 is also on course to enter the heliosheath, although at a different point along the solar system's edge. NASA scientists hope that both craft will eventually cross onto interstellar space.

But nothing lasts forever. Current power and propellant levels suggest that the Voyagers will no longer be able to collect and beam back data by 2020. At that point, the craft will be effectively lost in space.